Learn about the impacts of drought on freshwater ecosystems in the U.S. Caribbean below.
Author: Bonnie Myers, National Climate Adaptation Science Center; North Carolina Cooperative Fish and Wildlife Research Unit, Department of Applied Ecology, North Carolina State University
Click here to download the 2-page fact sheet with graphics, or read below for an extended version of the fact sheet.
Healthy and functioning freshwater ecosystems are needed for successful conservation and management of native fish and invertebrate species, and the services they provide to human communities, across the U.S. Caribbean. Yet streams, rivers, and reservoirs are vulnerable to the effects of extreme weather events, urbanization, energy and water development, and other environmental and human-caused disturbances (Neal et al., 2009). One major management concern is the impact of prolonged drought on freshwater ecosystems. Drought impacts streamflow, dissolved oxygen content, water quality, stream connectivity, available habitat, and other important freshwater habitat characteristics necessary for sustaining fish and invertebrate populations (Covich et al., 2006). These changes can impact species interactions, abundance, life history events, and the presence of native and non-native species (Larsen, 2000; Covich et al., 2006; Ramírez et al., 2018).
Drought impacts aquatic ecosystems and species both in the short-term and long-term, depending on the severity and duration of the event (e.g. Covich et al., 2006). In Puerto Rico, all native freshwater fish, shrimp, and snail species spend part of their lives in estuarine and marine ecosystems and depend on being able to move between these habitats to survive, so maintaining connectivity is key (e.g., Engman et al., 2017). Freshwater ecosystems also provide recreational, cultural, and ecological value to humans (Kwak et al., 2007; Neal et al., 2009). For example, some communities in Puerto Rico engage in artisanal shrimp and freshwater crab fishing (Neal et al., 2009). Artisanal fishing for postlarvae gobioids, known colloquially as “cetí” also occurs at the river mouths of large drainages and has strong cultural significance in parts of Puerto Rico, such as Arecibo (Kwak et al., 2016).
The U.S. Virgin Islands (USVI) is particularly sensitive to drought, because almost all streams are ephemeral and typically only flow after rainfall. These intermittent channels, known locally as “ghuts”, run down the surface of steep slopes, rather than through the ground, and are important sources of freshwater. Natural springs are often located in ghuts and can form pools of freshwater that serve as habitat for wetland and migratory birds, freshwater shrimp and fish, and amphibians (Nemeth and Platenburg, 2007; Gardner, 2008).
Drought conditions can negatively alter aquatic ecosystems, with immediate effects on fish and invertebrate species, such as fish kills in reservoirs.
- Drought conditions can greatly reduce stream flow, which impacts habitat quality and availability for freshwater organisms (Covich et al., 2006)
- Reduced flows can also impact important pool habitat and dissolved oxygen concentrations, which can impact fish and invertebrate growth and survival (Larsen, 2000).
- Reduced flows can result in lower numbers of migrating juveniles, which could negatively affect population size of amphidromous species and the cetí fishery (Engman et al., 2017). Reduced flow can also affect the transport of larvae from the river to the ocean (Collazo et al., 2018).
Changes in water quality: Shifts in pH, nutrient loads, concentration of contaminants, salinity, filtration capacity, temperature, and increases in aquatic algae can negatively impact freshwater habitat and organism health and survival (Neal et al., 2009; Nemeth and Platenberg, 2007).
Invasion of introduced species: Reduced discharge and frequency of floods could facilitate the invasion of introduced fish species in freshwater ecosystems, creating more competition for native individuals (Ramírez et al., 2018).
- Changes in fish assemblages: Increases in non-native species (Ramírez et al., 2018), shifts in the presence of native fish and invertebrate species (Ramírez et al., 2018), and permanent loss of crayfish-like species endemic to USVI, known as “creveshi” can occur.
- Species interactions: Alteration of food webs and species interactions (Covich et al., 2006)
- Habitat availability and connectivity: Restrictions on upstream migrations of fish and invertebrates, impacting their ability to carry out important life history events, such as reproduction (Covich et al., 2006).
Reductions in stream flow caused by severe drought can impact freshwater ecosystems island-wide (Ramírez et al., 2018). In Puerto Rico, rainfall varies across the island, with southwestern Puerto Rico experiencing less rain than the eastern part of the island. For example, during the 2015 drought, the eastern half of the island, which normally receives more rainfall, was under severe drought, while the drier, western half was under moderate drought or abnormally dry conditions (www.nrcs.usda.gov). The extreme 2015 drought resulted in a documented shift of native fish species to non-native species in an eastern Puerto Rican stream. Researchers concluded this was due to a decrease in discharge and reduction of flood events (Ramírez et al., 2018). This suggests that freshwater ecosystems that experience a shift in flooding frequency and decreased discharge during drought may be the most impacted as drought events become more frequent and severe with climate change.
Drought impacts on freshwater ecosystems can affect water supply for human consumption, fish and invertebrate populations, and agricultural practices (Larsen, 2000; Ramírez et al., 2018) Negative impacts on freshwater ecosystems, including reduction in water quality and frequency of flood events, can alter important habitat for fish and invertebrate species. Changes in freshwater organism assemblages can reduce the beneficial services that freshwater ecosystems provide to the public (Covich et al., 2006). In addition, public recreational use of aquatic resources could be affected by drought events, with socioeconomic and cultural impacts.
Current Activities & Future Research Directions
Current research activities:
- Continued research and management efforts by the Puerto Rico Department of Natural and Environmental Resources, U.S. Fish and Wildlife Service, U.S. Geological Survey, and Southeast Climate Adaptation Science Center to monitor stream flows, native and non-native populations, and habitat quality.
- Researchers with the University of Puerto Rico and North Carolina State University are monitoring and building on a long-term fish assemblage dataset from the Río Piedras, an urban river ecosystem in Puerto Rico.
- The Water Resources Research Institute at the University of the Virgin Islands is funding projects that address water resources issues in the USVI, such as those examining ecosystem integrity in riparian waterways and microclimate monitoring.
- Researchers and managers from universities, governmental, and non-governmental organizations continue to research the impacts of drought and other extreme weather events on fish and invertebrate species and freshwater ecosystem health and quality.
Key research needs:
- Long-term monitoring in both Puerto Rico and USVI, including monitoring before and after drought events to identify short- and long-term impacts.
- More accurate projections of future drought conditions for the region, to support adaptation planning and the development of appropriate, forward-looking management strategies.
- Improved understanding of the mechanisms behind drought impacts on species interactions and life histories.
- Increased engagement with water and energy management sectors to address flow standards.
Collazo, J. A., A. J. Terando, A. C. Engman, P. F. Fackler, and T. J. Kwak., 2018. Toward a resilience-based conservation strategy for wetlands in Puerto Rico—Meeting challenges posed by environmental change. Wetlands, 15 p. Access here.
Covich, A.P., Crowl, T.A., and Heartsill-Scalley, T., 2006, Effects of drought and hurricane disturbances on headwater distributions of palaemonid river shrimp (Macrobrachium spp.) in the Luquillo Mountains, Puerto Rico: Journal of the North American Benthological Society, v. 25, no.1, 99-107. Access here.
Engman, A. C., T. J. Kwak, and J. R. Fischer., 2017. Recruitment phenology and pelagic larval duration in Caribbean amphidromous fishes. Freshwater Science 36(4):851–865. Access here.
Gardner, L., 2008, A strategy for management of ghuts in the U.S. Virgin Islands: Water Resources Research Institute, University of the Virgin Islands. Access here.
Kwak, T.J., P.B. Cooney, and Brown, C.H., 2007, Fishery population and habitat assessment in Puerto Rico streams: phase 1 final report: Federal Aid in Sport fish Restoration Project F-50 Final Report, Submitted to Marine Resources Division, Puerto Rico Department of Natural and Environmental Resources, San Juan. Access here.
Kwak, T.J., Engman, A.C., Fischer, J.R., and Lilyestrom, C.G., 2016. Drivers of Caribbean freshwater ecosystems and fisheries. In: Freshwater, fish and the future: proceedings of the global cross-sectoral conference. (Eds W.W. Taylor, D.M. Bartley, C.I. Goddard, N.J. Leonard & R. Welcomme), pp. 219–232. Food and Agriculture Organization of the 139 United Nations; Michigan State University; and The American Fisheries Society, Rome; East Lansing; and Bethesda.
Larsen, M.C., 2000, Analysis of 20th century rainfall and streamflow to characterize drought and water resources in Puerto Rico: Physical Geography, v. 21, no. 6, 494-521. Access here.
Neal, J.W., Lilyestrom, C.G., and Kwak, T.J., 2009, Factors influencing tropical island freshwater fishes: species, status, and management implications in Puerto Rico: Fisheries, v. 34, no.11, 546-554. Access here.
Nemeth, D., and Platenberg, R., 2007, Diversity of freshwater fish and crustaceans of St. Thomas watersheds and its relationship to water quality as affected by residential and commercial development: Water Resources Research Institute Project 2006VI73B, University of the Virgin Islands, St. Thomas. Access here.
Ramírez, A., Gutiérrez-Fonseca, P.E., Kelly, S.P., Engman, A.C., Wagner, K., Rosas, K.G. and Rodríguez, N., 2018, Drought facilitates species invasions in urban streams: results from a long-term study of tropical island fish assemblage structure: Frontiers in Ecology and Evolution, v. 6, p.115. Access here.